1. Field of the Invention
This invention is directed to synthetic vitamin D derivatives which are characterized by 1-thio substitution, these vitamin D derivatives demonstrating biological activity. Included in the invention is the process for producing the biologically active vitamin D derivatives as well.
2. Brief Description of Background Art
The chemistry, biochemistry, physiology, and metabolism of vitamin D are well described by Holick et al., Harrison's PRINCIPLES OF INTERNAL MEDICINE, 10th Edition (1983), McGraw-Hill Book Company, New York, pages 1944-1948. Vitamin D is actually a hormone rather than a vitamin, and is a derivative of a cutaneous steroid precursor, 7-dehydrocholesterol (provitamin D.sub.3). Upon exposure to ultraviolet radiation, the precursor undergoes a photochemical reaction resulting in the cleavage of a C--C bond between C.sub.9 and C.sub.10, thereby generating a 9,10-secosteroid (previtamin D.sub.3). Previtamin D.sub.3 is a thermally labile intermediate, the compound undergoing molecular arrangement involving its conjugated triene system to a thermally stable 9,10-secosteroid, vitamin D.sub.3.
Vitamin D.sub.2 differs from vitamin D.sub.3 in that it contains a double bond between C.sub.22 and C.sub.23 and further contains a C.sub.24 -methyl group. And while vitamin D.sub.3 is the only endogenous form of vitamin D in human skin, both vitamins D.sub.2 and D.sub.3 are metabolized identically and have equivalent biological potency in most mammals. Vitamin D.sub.4 has a single bond between C.sub.22 and C.sub.23 and contains a methyl group at C.sub.24. As is generally understood, for the purposes of this disclosure, the term "vitamin D" is intended to include vitamin D.sub.2, vitamin D.sub.3, and vitamin D.sub.4.
Upon exposure to sunlight, the ultraviolet radiation energy between wavelengths 290-320 nm results in the photochemical conversion of 7-dehydrocholesterol (provitamin D.sub.3) stored in the epidermal layer to previtamin D.sub.3. Previtamin D.sub.3 slowly converts to vitamin D.sub.3 by a temperature-dependent process that takes approximately 48 hours. Once vitamin D enters the circulation, either by its absorption in the small intestine from the diet or through the skin, it is transported through the circulation by a specific transport protein, an alpha globulin, to the liver for its hydroxylation.
In the liver, vitamin D is metabolized to 25-hydroxyvitamin D by various vitamin D-25-hydroxylases, with 25-hydroxyvitamin D being one of the major circulating metabolites of vitamin D.
25-hydroxyvitamin D is not considered to be biologically active at physiological levels in vivo. After formation in the liver, 25-hydroxyvitamin D is bound by the high-affinity vitamin D-binding protein of plasma and is transported to the kidney for an additional stereospecific hydroxylation on either C.sub.1 or C.sub.24, the kidney playing a pivotal role in the metabolism of 25-hydroxyvitamin D to biologically active metabolites.
It is now recognized that the 1-alpha-hydroxy group is important in the binding of 1,25-dihydroxy vitamin D.sub.3 with its intestinal cytosolic receptor and in the expression of its calcium-transport activity (Norman, A. W., Vitamin D, Basic and Clinical Nutrition, Marcel Dekker, Inc., Vol. 2, 224-231 (1980); Franceschi, R. T. et al., Arch. Biochem. Biophys. 210, 1 (1981)).
1-alpha,25-dihydroxyvitamin D.sub.3 has been established as the hormonally active form of vitamin D.sub.3 (DeLuca, H. F. et al., Annu. Rev. Biochem. 45, 631 (1976); Norman, A. W. et al., Endocrin. Rev. 3, 331 (1982)). The 1-alpha, 25-dihydroxyvitamin D is taken up in the intenstine by specific cytoplasmic receptor proteins to stimulate calcium and phosphate transport from the intestinal lumen into the circulation. Vitamin D.sub.3 deficiency, or disturbances in the metabolism of vitamin D.sub.3 cause such diseases as rickets, renal osteodystrophy and related bone diseases, as well as, generally, hypo- and hyper-calcemic states. Thus, biologically active forms of vitamin D.sub.3 are crucial in maintaining normal development of bone structure by regulating blood calcium levels.
Inadequate endogenous production of vitamin D.sub.3 in the skin, insufficient dietary supplementation and/or the inability of the small intestine to absorb adequate amounts of vitamin D from the diet may cause hypovitaminosis D. Disease states which are equivalent to hypovitaminosis D are caused by drugs that antagonize vitamin D action, alteration in the metabolism of vitamin D, or inadequate response of tissue receptors to vitamin D metabolites. Hypovitaminosis D or equivalent disease states result in disturbances in the mineral ion metabolism and PTH secretion and mineralization defects in the skeleton such as rickets in children and osteomalacia in adults. As may be expected, adjunct therapy for hypovitaminosis D includes administration of vitamin D.sub.2, vitamin D.sub.3 or their metabolites which are hydroxylated at positions 1; 1,25; 1,24,25; 24,25; 25,26; or 1,25,26. However, these compounds are water insoluble, rendering post-administration dissolution in the gastrointestinal lumen as the rate-limiting step in drug absorption. As is disclosed in U.S. Pat. No. 4,410,515 to Holick, Sally A. et al., a form of vitamin D now exists which is hydrophilic and/or water-soluble, yet preserves the normal biological properties of the water-insoluble drug. These compounds, vitamin D glycosides, are derivatives of vitamin D which are characterized by the presence of a straight or branched chain glycosidic residue containing 1-20 glycoside units per residue substituted at the C.sub.1, C.sub.24, C.sub.25, and/or C.sub.26 positions. These vitamin D glycosides and their methods of production are fully disclosed in the aforementioned U.S. Pat. No. 4,410,515, the entire disclosure of which is incorporated by reference herein.
Alternative methods for the solubilization of vitamin D derivatives include conjugating compounds to glycosyl orthoester residues, as disclosed in co-pending U.S. patent application Ser. No. 607,117 of Holick et al., filed May 3, 1984, and U.S. patent application Ser. No. 667,813 to Holick et al., filed Nov. 2, 1984, both applications incorporated by reference herein.
Other derivatives of vitamin D are known as well, wherein the compounds contain a 5,6-epoxy (described in Jpn. Kokai Tokkyo Koho JP 58,216,178 [83,216,178], Dec. 15, 1983) and fluoro derivatives (the production of which is described in Shiina, et al., Arch. Biochem. Biophys. 220, 90 (1983)) and are further incorporated by reference herein.
It has also previously been reported that replacement of the 3-beta-hydroxy of vitamin D.sub.3 with a thio group has made the vitamin biologically inert (Bernstein, S. et al., J. Organ. Chem., 16, 685 (1951). 1-fluoro vitamin D.sub.3 has been been synthesized and shown to possess biological activity, possibly as a result of 25-hydroxylation (Napoli, J. L. et al., Biochemistry 18, 1641 (1979)). However, 1-alpha-25-difluoro vitamin D.sub.3 has been found to be biologically inert (Paaren, H. E. et al., Arch. Biochem. Biophys. 209, 579 (1981).
However, prior to the present, vitamin D derivatives containing 1-thio substitution and demonstrating biological activity have not been reported.